THE LUMINIPRROUS ETHER. 95 



than its vast or even possibly limitless extent. Matter of 

 which our senses give us any cognizance is heavy, that is to 

 say, it gravitates towards other matter wliich agrees with it 

 in so far as being accessible to oar senses. The question 

 presents itself to the mind, does the ether gravitate towards 

 what Ave call ponderable matter? This is a question to which 

 we are not able to give any positive scientific answer. It 

 the ether be in some way or other connected with the cause 

 of gravitation, it would seem more likely that it itself does 

 not gravitate towards ponderable matter. 



Again, we have very strong reason for believing that 

 ponderable matter consists of ultimate molecules. First, 

 that supposition accords in the simplest way with the laws 

 of crystallography. Chemical laws afford still stronger con- 

 firmation of the hypothesis, throiigh the atomic theory of 

 Dalton, now universally accepted. Comparatively recently, 

 the deduction of the fimdamental property of gases from 

 the kinetic theory, as it is called, affords strong additional 

 confirmation of tJiat view of the constitution of mattei". 

 Still more recently, the explanation which has been afforded 

 by that theory of that most remarkable instrument the radio- 

 meter of Crookes has lent further confirmation in the same 

 direction. None of these evidences apply to the ether, and 

 accordingly we are left in doubt whether it too consists of 

 ultimate molecules, or whether on the other hand it is con- 

 tinuous, as we cannot help conceiving space to be. 



The undulatory theory of light was greatly promoted in 

 the first instance by the known phenomena of sound, and 

 the explanation which they received from the hydrodynami- 

 cal theory. Accordingly, since sound, as we know, consists 

 of an undulatory movement propagated through the air (or 

 it may be through other media), and depending upon con- 

 densation and rarefaction, it was supposed naturally that 

 light was propagated in a similar manner, by virtue of the 

 forces brought into play by the condensation and rarefaction 

 of the ether. But there is one whole class of phenomena 

 which have actually no counterpart in those of sound; I 

 refer to polarization and double refraction. 



The evidence for the truth of the theory of undulations 

 as regards the phenomena of common light depends in 

 great measure upon the fact of interference and the expla- 

 nation which the theory gives of the complicated phenomena 

 of diffraction. But in studying the interference of polarized 

 light, additional plienomena presented themselves which 



